1. Field of the Invention
This invention relates to a zoom lens of the optical type called an inner-focus or a rear-focus type.
2. Description of the Related Art
During recent years, various focus adjusting methods of shifting a lens group other than a front lens group have been contrived for the purpose of increasing the efficiency and magnification of the zoom lens.
Meanwhile, the zoom lenses have become automatically focusable in general. Demands have increased very much for reduction in weight of a focusing lens and for the zoom lens of the inner-focus or rear-focus type having its movable part arranged to be untouchable directly from the outside.
The known zoom lenses of this kind include:
(a) Zoom lenses of the type having the locus of zooming movement of the focusing lens group arranged to serve also as a part of the focusing movement of the focusing lens group, as disclosed, for example, in U.S. Pat. No. 4,534,625; Japanese Laid-Open Patent Application No. SHO 63-304217 (corresponding U.S. Pat. unknown); Japanese Laid-Open Patent Application No. SHO 64-35515 (corresponding U.S. Pat. unknown); and Japanese Laid-Open Patent Application No. SHO 64-35516 (corresponding U.S. patent unknown).
(b) Zoom lenses of the type having the focusing cam locus arranged to be formed separately from the zooming cam locus, as disclosed in Japanese Laid-Open Patent Application No. SHO 63-49715 (U.S. Pat. unknown).
(c) Zoom lenses which are provided with no cam that rotates in a fixed position, as disclosed in Japanese Laid-Open Patent Application No. SHO 63-314511 (U.S. Pat. unknown).
However, these prior art zoom lenses have presented the following problems:
The zoom lenses of the type of Para. (a) above are arranged to have the zooming movement of the focusing lens group to serve also as a movement to be made in focusing. The degree and direction of focusing rotation in relation to the angle of zooming rotation are thus determined in the designing stage of the lens. Therefore, the degree of latitude allowable for them is limited.
In the case of the zoom lenses of the type of Para. (b), a manner in which a zooming cam ring and a focusing cam ring are interconnected in the direction of rotation differs from a manner in which they are interconnected in the direction of the optical axis. Hence, they tend to be affected by the precision of parts and their fitting play.
The zoom lenses of Para. (c) are arranged to have a zooming cam shift its position in zooming back and forth non-linearly in the direction of the optical axis. It is, therefore, difficult. to keep a sufficient degree of precision for a lens group which is arranged to move in zooming. This also necessitates some protective measures against impact if a first lens group is arranged to be the lens group that moves in zooming. Therefore, efforts to reduce the size of the lens has been restricted.